Microgrid Basics


Published on:

2020-03-31

Distributed energy sources (DER): Generally defined as including distributed generation (DG), energy storage devices (ES), and systems connected to the public grid. DG refers to the small power generation system connected to the user side to meet the special needs of end users, mainly including internal combustion engines, micro gas turbines, fuel cells, solar energy, wind energy and other power generation;

1. Definition and distinction
Distributed energy sources (DER): Generally defined as including distributed generation (DG), energy storage devices (ES), and systems connected to the public grid. DG refers to the small power generation system connected to the user side to meet the special needs of end users, mainly including internal combustion engines, micro gas turbines, fuel cells, solar energy, wind energy and other power generation;
Micro-Grid: A small power generation and distribution system composed of distributed power sources, energy storage devices, energy conversion devices, loads, monitoring and protection devices, etc. A microgrid is an autonomous system capable of self-control, protection, and management, and can operate either in conjunction with an external grid or in isolation.
2. The structure of the microgrid
 
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3. The architecture of the microgrid
The architecture of the microgrid generally adopts the internationally mature three-layer structure (the same is true of Xuji's demonstration project): the distribution network dispatch layer, the microgrid centralized control layer, the distributed power source and the load local control layer.
 
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4. Control strategy of microgrid
Microgrid overall control strategy
The control strategies commonly used in microgrid are mainly divided into two types, master-slave operation and peer-to-peer control.
Master-slave control mode: One distributed power source (or energy storage device) in the microgrid adopts V/f control to provide voltage and frequency reference for the microgrid, while other distributed power sources use PQ control.
Peer-to-peer control mode: Multiple controllable distributed power sources (or energy storage devices) participating in voltage and frequency regulation and control in the microgrid have equal status in control. Usually, P-f and Q-V droop control methods are selected. The local information of the power access point is controlled.
Control Strategy of Micro Power Supply
Because the microgrid is based on power electronic technology, the micropower supply in it has high controllability. The control strategies for the micropower supply can be divided into three categories: constant power control, droop control and constant voltage and constant frequency control.
Constant power control (PQ control): Realize the control of active and reactive power = reference value. When distributed generation systems such as photovoltaics and wind turbines use maximum power tracking control, they belong to constant power control. When the microgrid is connected to the grid, the grid provides voltage and frequency reference, and each distributed power source generally adopts constant power control. Of course, some controllable distributed power sources can also use the f-P and V-Q droop control methods, which can support the grid voltage and frequency when the grid voltage amplitude and frequency decrease.
Constant voltage/constant frequency control: Control voltage and frequency.
Droop control: select the frequency primary droop characteristic curve (DroopCharacter) similar to the traditional generator as the control method, namely obtain stable frequency and voltage through P/f droop control and Q/V droop control respectively. The active power and reactive power output by the micro-sources in the micro-grid are controlled separately, without the need for communication and coordination between units, realizing the goal of plug-and-play and peer-to-peer control of the micro-sources.
 
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5. Two operating modes of microgrid
There are two typical operating modes of the microgrid: under normal circumstances, the microgrid is connected to the grid with the conventional distribution grid, which is called the networking mode; when a grid failure is detected or the power quality does not meet the requirements, the microgrid will be disconnected from the grid in time. And independent operation, called island mode. Switching between the two must be smooth and fast. Compared with the external large power grid, the microgrid behaves as a single controlled unit, and can meet the user's requirements for power quality and power supply security at the same time. The power supply inside the microgrid is mainly responsible for the energy conversion by power electronic devices and provides the necessary control. (1) Grid-connected operation: The microgrid is connected to the public large grid, and the microgrid circuit breaker is closed to exchange electrical energy with the main grid distribution system. The photovoltaic system is connected to the grid for power generation. The energy storage system can perform charging and discharging operations in grid-connected mode. During grid-connected operation, it can be switched to off-grid operation through the control device.
(2) Off-grid operation: also known as island operation, it refers to the operation mode composed of DG, energy storage device and load when it is disconnected from the main grid distribution system when the power grid fails or is required by the plan. The energy storage converter PCS works in the off-grid operation mode to continue supplying power to the microgrid load. The photovoltaic system continues to generate electricity due to the restoration of power supply from the busbar, and the energy storage system usually only supplies power to the load.
6. Microgrid planning
 
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